The Impacts of Agriculture on the Environment: Atmosphere

Last week we talked about the dramatic amount of land and water used for agriculture. However, other than land and water, it also largely affects the atmosphere. People typically attribute greenhouse gas (GHG) emission to energy production, what they do not realize is that agriculture is also one of the biggest emitters of greenhouse gases which make it one of the largest contributors of global warming. 

Agriculture's Contribution to Greenhouse Gas Emission 

The most recent IPCC report (2014) state that agriculture, forestry and land use (AFOLU) was responsible for 24% of direct global GHG emission, which is even more than industry emission of 21% and transportation of 14%. 

The chart below shows the total greenhouse gas emissions by sector. The combined percentage of agriculture, forestry and land use sources shows a similar result to that of the IPCC: collectively they account for 11.8 million GgCO2-eq (=11.8 GtCO2-eq) which is approximately one-quarter of the total anthropogenic emissions of GHGs. 

Fig.1 Global greenhouse gas emissions by sector (Source: OurWorldInData)
Data is based on UN reported figures, sourced from the EDGAR database. 

To be specific (IPCC):

• Carbon dioxide (CO2) accounts for 21-25% if total CO2 emission;
• Methane (CH4) accounts for 55-60% if total CH4 emission;
• Nitrous oxide (N2O) accounts for 65-80% if total N2O emission.

Emission sources and Potential for Control

These emissions are generally linked to the management of agriculture soil, livestock, rice production and biomass burning. 

CO2 emission comes from the direct human-induced impact on forestry and land use, such as deforestation and shifting patterns of cultivation and also fossil fuels used on farms. 
At the same time, land can act as carbon sinks through reforestation, soil improvements and other activities. 

CH4 comes mainly from livestock breeding. Enteric fermentation by ruminant animals such as cattle, sheep and goats, produces CH4 as a by-product as part of their normal digestive processes. As our agriculture reliance in most countries is on these animals, it is one of main anthropogenic methane production sources.  Since this process is not 100% efficient, some of the food energy is lost in form of methane, ranging from 2-12% of gross energy intake. Therefore, mitigating enteric fermentation by decreasing enteric CH4 production without altering animal productivity is a desirable strategy to reduce GHG emission and improving digestive efficiency (Martin et al., 2009). The other major contribution of CH4 emission is from rice cultivation, which is the staple food for more than half the world’s population. The warm, waterlogged soil in paddy fields provide ideal conditions for methane-producing microscopic organisms. On average, the paddy soil is only fully waterlogged for around 4 months each year, so for the rest of time, the methanogenesis is considerately less. Moreover, when the soil dries out, it can be a temporary sink for atmospheric methane.  Therefore, it is considered that any farming method that reduces the period of flooding could reduce methane. 

Nitrogenous fertilizers cultivated soils are the primary source of N2O emissions, including synthetic fertilizers such as urea or anhydrous ammonia, or organic fertilizers such as manure. As most fertilizer nitrogen is mobile, it is hard to contain in soil and susceptible to loss when it is not taken up by plants. It can be lost as nitrates to groundwater or as gases N2O, N2, ammonia to the atmosphere. Cassman et al. (2014) found that typically only half of the applied nitrogen is taken up by crops during the growing season. The management of better targeting the fertilizer application in both time and space can prevent the build-up of nitrogen and significantly reduce N2O emission.